1. Field of the Invention
The present invention relates to an ultraviolet ray-shielding agent and tube. More particularly, the present invention relates to an ultraviolet ray-shielding agent and tube for a discharge lamp.
2. Description of the Related Arts
It is known that various types of luminescent lamps, for example, mercury lamps, metal halide vapor lamps, sodium lamps, xenon lamps and halogen lamps, have an excellent luminance and brightness, a high illumination efficiency, and a long durability, and thus are useful for illuminating buildings such as shops, and as fish-luring lamps.
Luminescent lamps irradiate strong ultraviolet rays in addition to visible rays, and due to recent increases in the luminance or brightness of the luminescent lamps, for example, halogen lamps, the ultraviolet ray-radiation therefrom can no longer be ignored. Namely, when these lamps are used to illuminate, for example, department stores, the ultraviolet rays cause a discoloration and deterioration of the goods, and when used as fish-luring lights, the ultraviolet rays burn the skin of the users and may cause skin cancer or a deterioration of the eyesight of the users. Therefore, it is necessary that some form of shielding from the ultraviolet rays emitted by luminescent lamp be provided.
Several attempts have been made to shield the ultraviolet rays, as shown in the following description:
(1) In a metal vapor luminescent lamp, a coating layer of a titanium dioxide is formed on an outside or inside surface of a tube or bulb in which a luminescent source is contained.
(2) In a xenon lamp, a tube or bulb for containing a luminescent source is made from a transparent quartz containing 10 to 300 ppm of at least one member selected from titanium dioxide and cerium oxide.
Nevertheless, in the above-mentioned ultraviolet ray-shielding tubes, titanium dioxide or cerium oxide is utilized as the ultraviolet ray-shielding material, but these compounds are disadvantageous in that they have the following properties:
(1) A high refractive index and a poor transparency, and thus the luminance of the luminescent lamp is lowered.
(2) When titanium dioxide is used in the form of fine particles, the particles absorb visible rays, and thus the luminance or brightness and color-rendering property of the lamp are lowered, due to a relatively large size of these particles.
(3) When an organic titanium compound is employed, it is difficult to form a coating layer having a large thickness, and thus the resultant coating layer exhibits an unsatisfactory ultraviolet ray-shielding property.
(4) The transparent quartz containing titanium dioxide or cerium oxide is expensive. Also there is an upper limit to the amount of titanium dioxide or cerium oxide that can be added to the quartz, and thus the ultraviolet ray-shielding property of the resultant quartz tube or bulb is still unsatisfactory.
Generally, ultraviolet ray-shielding coating materials must satisfy all of the following requirements:
(1) The coating material must be able to shield ultraviolet rays at a wave length of around 400 nm or less.
(2) The coating material must be stable for practical use over a long period.
(3) The coating material must be harmless to the human body.
The conventional ultraviolet ray-shielding agents however, cannot satisfy all of the above-mentioned requirements.